Play either the G major scale or the G major pentatonic scale (which you may notice is just like the major scale but fewer notes). These
diagrams also give you a choice depending whether you have a high G (more usual) or a low G uke.
Have a few goes through then try (at your own speed) with the G major backing track - click on the track in the list and it will start to play (after one bar rest and a
count-in).